Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 15
Biogeosciences, 13, 4513-4532, 2016
https://doi.org/10.5194/bg-13-4513-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 13, 4513-4532, 2016
https://doi.org/10.5194/bg-13-4513-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 12 Aug 2016

Research article | 12 Aug 2016

Low Florida coral calcification rates in the Plio-Pleistocene

Thomas C. Brachert1, Markus Reuter2, Stefan Krüger1, James S. Klaus3, Kevin Helmle4, and Janice M. Lough5 Thomas C. Brachert et al.
  • 1Institut für Geophysik und Geologie, Universität Leipzig, Leipzig, Germany
  • 2Institute of Earth Sciences, NAWI Graz Geocentre, University of Graz, Graz, Austria
  • 3Department of Geological Sciences, University of Miami, Coral Gables, Florida, USA
  • 4Oceanographic Center, Nova Southeastern University, Fort Lauderdale, Florida, USA
  • 5Australian Institute of Marine Science, Townsville MC, Australia

Abstract. In geological outcrops and drill cores from reef frameworks, the skeletons of scleractinian corals are usually leached and more or less completely transformed into sparry calcite because the highly porous skeletons formed of metastable aragonite (CaCO3) undergo rapid diagenetic alteration. Upon alteration, ghost structures of the distinct annual growth bands often allow for reconstructions of annual extension ( = growth) rates, but information on skeletal density needed for reconstructions of calcification rates is invariably lost. This report presents the bulk density, extension rates and calcification rates of fossil reef corals which underwent minor diagenetic alteration only. The corals derive from unlithified shallow water carbonates of the Florida platform (south-eastern USA), which formed during four interglacial sea level highstands dated approximately 3.2, 2.9, 1.8, and 1.2Ma in the mid-Pliocene to early Pleistocene. With regard to the preservation, the coral skeletons display smooth growth surfaces with minor volumes of marine aragonite cement within intra-skeletal porosity. Within the skeletal structures, voids are commonly present along centres of calcification which lack secondary cements. Mean extension rates were 0.44±0.19cmyr−1 (range 0.16 to 0.86cmyr−1), mean bulk density was 0.96±0.36gcm−3 (range 0.55 to 1.83gcm−3) and calcification rates ranged from 0.18 to 0.82gcm−2yr−1 (mean 0.38±0.16gcm−2yr−1), values which are 50% of modern shallow-water reef corals. To understand the possible mechanisms behind these low calcification rates, we compared the fossil calcification rates with those of modern zooxanthellate corals (z corals) from the Western Atlantic (WA) and Indo-Pacific calibrated against sea surface temperature (SST). In the fossil data, we found a widely analogous relationship with SST in z corals from the WA, i.e. density increases and extension rate decreases with increasing SST, but over a significantly larger temperature window during the Plio-Pleistocene. With regard to the environment of coral growth, stable isotope proxy data from the fossil corals and the overall structure of the ancient shallow marine communities are consistent with a well-mixed, open marine environment similar to the present-day Florida Reef Tract, but variably affected by intermittent upwelling. Upwelling along the platform may explain low rates of reef coral calcification and inorganic cementation, but is too localised to account also for low extension rates of Pliocene z corals throughout the tropical WA region. Low aragonite saturation on a more global scale in response to rapid glacial–interglacial CO2 cyclicity is also a potential factor, but Plio-Pleistocene atmospheric pCO2 is generally believed to have been broadly similar to the present day. Heat stress related to globally high interglacial SST only episodically moderated by intermittent upwelling affecting the Florida platform seems to be another likely reason for low calcification rates. From these observations we suggest some present coral reef systems to be endangered from future ocean warming.

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We have analysed the rate of calcification of fossil reef corals. These measurements are important, because the rate of formation of the skeleton depends on the physical environment in which the corals lived. The rates of skeletal calcification of the fossils were approximately 50 % lower than they are in extant reef corals. This is a likely effect of high water temperatures and/or low carbonate saturation of the water – factors that will also affect coral growth by future global warming.
We have analysed the rate of calcification of fossil reef corals. These measurements are...
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